The present invention relates generally to methods for determining physiological characteristics. More specifically, the present invention relates to a method and apparatus for determination of physiological characteristics that employs a plurality of measured physiological signals and an individualized reference signal.
Non-invasive photoelectric pulse oximetry for determining blood flow characteristics is well known in the art. Illustrative are the methods and apparatus described in U.S. Pat. Nos. 5,490,505; 4,934,372; 4,407,290; 4,226,554; 4,086,915; 3,998,550; and 3,704,706.
Pulse oximeters typically measure and display various blood flow characteristics including, but not limited, to blood oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the flesh and the rate of blood pulsations corresponding to each heartbeat of the patient. The oximeters pass light through human or animal body tissue where blood perfuses the tissue such as a finger, an ear, the nasal septum or the scalp, and photoelectrically sense the absorption of light in the tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light passed through the tissue is typically selected to be of one or more wavelengths that is absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption.
The output signal from the pulse oximeter, which is sensitive to the arterial blood flow, contains a component that is waveform representative of the patient""s blood gas saturation. This component is referred to as a xe2x80x9cplethysmographic wave or waveformxe2x80x9d (see FIG. 5).
A problem generally associated with non-invasive pulse oximeters is that the plethysmograph signal (and the optically derived pulse rate) may be subject to irregular variants in the blood flow including, but not limited to, motion artifacts, that interfere with the detection of the blood flow characteristics. A motion artifact is caused by the patient""s muscle movement proximate to the oximeter sensor, for example, the patient""s finger, ear or other body part to which the oximeter sensor is attached, and may cause spurious pulses that are similar to pulses caused by arterial blood flow. These spurious pulses, in turn, may cause the oximeter to process the artifact waveform and provide erroneous data. This problem is particularly significant with infants, fetuses, or patients that do not remain still during monitoring.
A further problem exists in circumstances where the patient is in poor condition and the pulse strength is very weak. In continuously processing the optical data, it can be difficult to separate the true pulsatile component from artifact pulses and noise because of a low signal to noise ratio. Inability to reliably detect the pulsatile component in the optical signal may result in a lack of the information needed to calculate blood constituents.
Several signal processing methods (and apparatus) have been employed to reduce the effects of the motion artifact(s) on the measured signal(s) and, hence, derived plethysmograph waveform. For example, in U.S. Pat. No. 4,934,372 synchronous averaging (i.e., xe2x80x9cC-lock techniquexe2x80x9d) is employed to eliminate the motion artifact(s).
Although the noted method has been embodied in at least one commercially available device, there are several drawbacks associated with the method. Among the drawbacks is that the noise reduction is equal to approximately 1/n, where n equals the number of heartbeats in the synchronous average. The value (n) could thus cause inordinate delays in the determination of blood constituents (e.g., blood saturation).
In U.S. Pat No. 5,490,505 a complex method and apparatus is disclosed that employs a plurality of signal conditioners, a combining network and a correlation canceler to address the xe2x80x9cmotion artifactxe2x80x9d issue. There are several drawbacks associated with the noted method and apparatus.
A major drawback of the ""505 method and apparatus is that no noise free reference or signal free noise source is provided to the correlation canceler. The derived plethysmographic waveform may thus not be representative of the patient""s true plethysmographic waveform.
The noted method is further based on the questionable assumption that the maximum correlation value (xcfx89) provided by the combining network at the highest saturation is the arterial saturation and the maximum correlation value at the lowest saturation is the venous saturation. It is, however, well known in the art that there are a number of conditions, especially unhealthy conditions, where the motion artifacts will produce a saturation value that is higher than the arterial saturation. Indeed, a pure motion artifact with no blood in the path at all will produce a blood saturation of approximately 81%.
It is therefore an object of the present invention to provide an improved method and apparatus for determining physiological characteristics of a subject and, in particular, the blood oxygen saturation level in the subject""s blood.
It is another object of the invention to provide a method and apparatus for determining physiological characteristics of a subject that provides an accurate representation of the subject""s plethysmographic waveform.
In accordance with the above objects and those that will be mentioned and will become apparent below, the method for determining physiological characteristics and, in particular, the blood oxygen saturation level of a subject in accordance with this invention comprises (a) acquiring a first blood oxygen signal from the subject, the blood oxygen signal having an undesirable artifact signal component; (b) acquiring an additional physiological signal having a heart rate component using an acquisition technique that is different and independent from the first acquiring step; (c) processing the first blood oxygen signal and the physiological signal to provide a first waveform having a reduced level of the artifact signal component therein; (d) processing the first waveform and the physiological signal to provide a reference waveform; and (e) processing the reference waveform and the physiological signal to provide a second blood oxygen signal corresponding to the blood oxygen saturation level of said subject.
The apparatus for determining the blood oxygen saturation level of a subject in accordance with the invention comprises an oximeter sensor arrangement coupled to the subject for acquiring a first blood oxygen signal having an artifact component; an ECG detector coupled to the subject for acquiring an ECG signal having a plurality of first r-wave components substantially corresponding to first r-wave events; first processing means for processing the first blood oxygen signal and the ECG signal to provide a first waveform having a reduced level of the artifact component, the first waveform including a plurality of quasi-stationary heartbeat spaces; second processing means for processing the first waveform and the ECG signal to provide a second waveform having said plurality of quasi-stationary heartbeat spaces and a plurality of second r-wave events substantially corresponding to said first r-wave events; and third processing means for processing the first blood oxygen signal and the second waveform to provide a second blood oxygen signal substantially corresponding to the blood oxygen saturation level of said subject.